The present invention relates to a projection device for the holographic reconstruction of scenes, said device comprising a spatial light modulator, an imaging system with at least two imaging means and an illumination system with at least one light source to generate sufficiently coherent light for the illumination of a hologram encoded on the light modulator. This invention further relates to a method for the holographic reconstruction of scenes.
Known 3D displays or 3D projection devices and methods usually take advantage of the stereo effect, where the light which generates the stereo impression is reflected on or emitted from a plane. However, in holography the light which is emitted by the hologram interferes in the object points of the scene, from where it propagates naturally. Holographic representations are object substitutions. In contrast, any forms of stereoscopic representations of unmoving (stills) or moving scenes do not represent object substitutions. They rather provide two plane images, one for the left and one for the right eye, where said images correspond with the two eye positions. The three-dimensional effect is created by the parallax in the two pictures. In a holographic representation, the problems known in conjunction with stereoscopy, such as fatigue, eyestrain and headache, do not occur, because there is generally no difference between viewing a real scene and a holographically reconstructed scene.
In holography it is generally distinguished between static and dynamic methods. In static holography, photographic media are predominantly used for information storage. This means that a reference beam is superimposed by a light beam which carries the object information such to record an interference pattern on the photographic medium. Such static object information is reconstructed with the help of a beam similar or identical to the reference beam. However, for example the entertainment industry or medical and military equipment manufacturers have been interested for a long time in a real-time representation of moving scenes using dynamic holography, because of the ideal spatial properties of such reconstructions. In most cases, micro displays of the same type as used in projection devices are employed. Micro displays can be, for example, liquid crystal on silicon (LCoS) panels, transmissive LCD panels or micro electro-mechanical systems (MEMS). Because their distance between the pixel centres, the pixel pitch, is small compared to other displays, a relatively large diffraction angle is achieved. A major disadvantage of hitherto known dynamic holographic methods which involve micro displays, however, is that the size of the reconstructions or of the reconstructed scenes is greatly limited by the size of the micro displays. Micro displays and similar light modulators have a size of few inches and, despite the relatively small pitch, still a diffraction angle which is so small that viewing a scene with both eyes is hardly possible. A very small pitch of only 5 μm, for example, results in a diffraction angle of about 0.1 rad at a wavelength λ of 500 nm (blue-green). At an observer distance of 50 cm, a lateral dimension of 5 cm is achieved, which does not allow the scene to be viewed with both eyes.
For a three-dimensional representation of dynamic holograms, typically computer-generated holograms, holographic reconstruction devices take advantage of transmissive or reflective light modulators, such as TFT, LCoS, MEMS, DMD (digital micro-mirror device), OASLM (optically addressed spatial light modulators), EASLM (electronically addressed spatial light modulators) and FLCD (ferro-electric liquid crystal displays) etc. Such light modulators can be of one- or two-dimensional design. The reasons why reflective light modulators are used are an inexpensive manufacturing process, a large fill factor for great light efficiency, short switching delays and only little light loss caused by absorbance compared with transmissive displays. However, the smaller spatial dimensions must be put up with.
WO 03/060612 describes a reflective LC display with a resolution of about 12 μm and a reflectance of up to 90% for real-time colour reconstruction of holograms. The reconstruction is carried out using the collimated light of one or multiple LEDs through a field lens. With this resolution, viewing is only possible in a region which is just about 3 cm wide at a distance of about 1 m, which is insufficient for the reconstructed scene to be viewed simultaneously with both eyes, i.e. in a three-dimensional way. Further, only relatively small objects can be reconstructed because of the small dimensions of the display.
WO 02/095503 discloses a holographic 3D projection device which uses a DMD chip for hologram reconstruction. However, despite the relatively high resolution, great reflectance and low switching delays of the light modulator, this device also only allows scenes with a small size to be reconstructed and to be viewed in a very small region for the same reasons mentioned in conjunction with WO 03/060612. The reason for this is again the small reconstruction space, which is defined by the dimensions of the light modulator and visibility region. Furthermore, DMD chips only partly suit holographic purposes due to their limited coherence.
WO 00/75699 discloses a holographic display which reconstructs a video hologram with the help of sub-holograms. This method is also known as tiling. Sub-holograms which are encoded on a very fast electronically addressable spatial light modulator (EASLM) are sequentially imaged into an intermediate plane. This process is executed at a high speed such that a observer perceives the reconstructions of all sub-holograms as a single reconstruction of a 3D object. The sub-holograms are arranged in a matrix structure in the intermediate plane by a specially designed illumination and imaging system, for example including a shutter which is controlled in synchronism with the EASLM and which only allows the corresponding sub-hologram to be transmitted and which in particular blocks unused diffraction orders. However, the demands made on the dynamic properties of the SLM used for reconstructing the sub-holograms are high, and a flat design does not appear to be feasible.
The above-mentioned solutions have the following major disadvantages in common. The spatial extension of the reconstruction is limited by the small size of the light modulators used for hologram reconstruction. The tiling method described in WO 00/75699 generally allows large scenes to be reconstructed, but this requires a voluminous design of the device. Because of the large number of pixels used, the computational load required to compute the hologram and the demands made on the data transfer rate will increase substantially, which makes it rather difficult to achieve real-time reconstructions. When using the sequential tiling method, as known from WO 00/75699, great demands are made on the dynamic properties of the SLM used.